This invention relates to a multi-fuel engine control for a vehicle engine having a fuel tank with a liquid fuel mixture comprising first and second combustible fuels, such as gasoline and methanol, with different volumetric heat contents. A fuel composition sensor may be responsive to a physical parameter of the fuel mixture to generate a fuel composition signal indicative of the relative proportions of the first and second fuels in the fuel mixture; and the engine, which includes induction means effective to provide the liquid fuel mixture and air to the engine in a desired air/fuel ratio, may be responsive to the fuel composition signal to derive therefrom a fuel control parameter and to determine from the fuel control parameter a fuel quantity required for the desired air/fuel ratio and thus compensate for the varying volumetric heat content of the fuel mixture with varying composition.
However, there are modes of engine operation which are quite stable from the standpoint of fuel control but in which the stability may be upset by repeated small changes due to the fuel composition signal. These changes may occur as a result of digitizing errors and electrical noise in the fuel composition signal as provided to the engine control computer. The mode of engine operation in which these changes may be undesirable will generally include closed loop fuel control at a stoichiometric mixture in response to an exhaust mounted air/fuel ratio sensor. If the open loop fuel control portion of the induction apparatus provides a minimum fuel change equivalent to 0.1 air/fuel ratios for a fuel composition sensor input change, the closed loop control may spend a considerable portion of the time chasing and trying to compensate for the open loop portion of the system. Although these problems might be overcome by the use of computer control hardware having more power and resolution, this involves, at this time, considerable additional expense which may be disproportionate to the problem.